1 Field of Invention
This invention relates to battery charging systems. Specifically, the present invention relates systems and methods for charging a cellular telephone battery over a cable and interface that have linear or nonlinear properties.
2 Description of the Related Art
As the demand for cellular telephones and general consumer electronics increases so does the need for efficient batteries, power supplies, and battery charging mechanisms. Efficient charging mechanisms are particularly important for batteries used in cellular telephones where the batteries are often charged daily.
The batteries of cellular telephones are often charged via an accessory that enables use of the phone while it is being charged. The accessory may include a charger that is connected to a power source in a car horn or other location and that supplies current to a battery in a cellular telephone via a cable. Additional electronic interfaces are often located in the phone between the cable and the phone battery. The charger supplies current to the battery until the voltage drop across the battery, as measured from the output terminals of the charger, reaches a predetermined voltage indicative of a charged battery. Many conventional charging power supplies however, fail to account for voltage drops across the cable and any electronic interfaces. As a result, such power supplies typically fail to fully charge the battery.
Many newer battery charging circuits attempt to account for the voltage drop across the cable and electronic interfaces by incorporating an additional constant voltage factor representative of an estimated voltage drop across the cable and electronic interfaces. For example, if the estimated voltage drop is 0.2 volts and the battery reads 4.0 volts when it is fully charged, the charging circuit will charge the battery until the voltage between the charging circuit terminals is approximately 4.2 volts. However, this method may result in an overcharging of the battery if the estimated voltage drop is larger than the actual voltage drop.
The voltage drop across the cable and any additional electronic interfaces is often a nonlinear function of current and temperature. As a result, the estimated voltage drop, i.e., the constant voltage factor, quickly becomes inaccurate as the current from the power supply changes or as the temperature of the cable and interface changes. This may result in an overcharging or an undercharging of the phone battery.
Alternatively, power supplies may contain control circuits that reduce current flowing through the cable as the battery becomes fully charged. By reducing the current flowing through cable and any associated interface, the voltage drop across the cable and interface is reduced, which reduces charging error caused by the voltage drop. However, as the current flowing through the cable and interface decreases, the time required to fully charge the battery increases. For example a standard desktop charger having no cables or electronic interface between the battery and charging terminals may take two hours to fully charge a phone battery, while a comparable charging accessory that charges the battery through linear or nonlinear cables or phone interfaces may take four hours. The additional required charging time is an inconvenience and may still result in an overcharging of the battery.
Hence, a need exists in the art for a system and method for quickly and accurately charging a battery while accounting for any voltage drop across linear or nonlinear circuitry between the charger and the battery.